An apparatus for chemical mechanical polishing (CMP) typically comprises a rotation table, on which a polishing pad conventionally made of polyurethane is mounted. A rotatable polishing head holds the wafer, which is to be polished, and engages the wafer against the rotating wetted polishing pad. During polishing, the polishing head, which either co-rotates or counter-rotates with the polishing pad, can vary its position relative to the axis of the rotation table due to an oscillating arm. Thereby, the textured polishing pad surface receives a so-called slurry which serves for abrading the wafer surface.
The slurry typically contains particles of, e.g., aluminum oxide or silicon dioxide in de-ionized water with a variety of chemical alloys to chemically oxidate and mechanically abrade surface material. By use of those chemical alloys, a high selectivity of the polishing rates of, e.g., polysilicon or tungsten against silicon dioxide can be maintained during planarization.
The abrasion rate depends on the respective rotation velocities of the tables and heads, the slurry concentration and the pressure with which the polishing head is engaged against the polishing pad.
Generally, polishing pads are affected by deterioration, since the uniform, textured and profiled pad surface is obliterated with removed wafer surface material, chemically altered slurry material, or deteriorated pad surface material, which is often referred to as the “pad glazing effect”.
This effect can be remedied by performing a conditioning step in which glass-like material as well as the uppermost deteriorated pad layer is removed from the pad by means of a conditioning disk. Thereby, the pores, which are to receive the slurry, are re-opened resulting in a restored pad functionality.
The process of conditioning can be carried out either during or after the polishing step. In one example, diamond emery paper is mounted on a conditioning head, which analogously to the polishing head, is carried by an additional oscillating arm. Diamond particles are encapsulated in a nickel grit mounted on a socket layer. The diamond particles are protruding from the nickel surface to various extents—ranging from being fully encapsulated to just being slightly stuck to the nickel layer.
The so structured conditioning pad grinds over the resilient polyurethane or similar polishing pad surface in a rotation movement of the conditioner, which is engaged onto the polishing pad. After the conditioning step, the efficiency of abrading is substantially restored, resulting in a prolonged lifetime of the pad and less operator efforts to replace deteriorated pads. Nevertheless, due to removing 0.2–1.5 μm of pad material thickness per conditioning cycle, the improved lifetime of the polishing pad due to performing the conditioning step is limited to, e.g., 12–18 hours, after which the polishing pad being mounted to the rotation table by adhesive means is to be replaced by a new one. Typically, the conditioning cycle performed after each wafer polishing needs 40–60 seconds of time. A typical polishing cycle takes about 2 minutes. Only roughly 500–1000 wafers can be polished with one pad, nowadays. New polishing pads typically start with 1–2 mm thickness. The lifetime of a pad depends on factors such as thickness removal and homogeneity, etc.
In polishing semiconductor wafers, a high degree of uniformity is needed in order to remove surface material under precisely determined removal rates. Non-uniformity can inevitably lead to specification violation of layer thicknesses and thus to a disadvantageous decrease in yield of the polishing process.
One cause for this non-uniformity can often be derived from a development of a polishing pad profile in an advanced state of conditioning. Under normal conditions, the centre and the edge areas of a polishing pad mounted on a polishing platen are neither affected by polishing nor by conditioning. Therefore, pad surface areas in the vicinity of the central area or the edges are less strongly affected by conditioning removal of surface material than the area having a radius within both limits, resulting in a significant slope of the polishing pad thickness profile. Additionally, material inhomogeneities can lead to local elevations affecting the wafer surface during polishing.
In prior art, this problem has been addressed by removing the polishing pad from the polishing platen and performing a high-precision thickness profile measurement using, e.g., a micrometer. This procedure is disadvantageously connected with time consuming profile measurements as well as the handicap of destroying the polishing pad under investigation, such that the measurement result cannot directly be reused for the current pad. Rather, a trend can be estimated of how efficient the current conditioning is, and how long a pad can generally be used until its lifetime ends. If, alternatively, problems with the CMP-apparatus quickly evolve, too many wafers are disadvantageously processed until the problem is found and a reaction can be taken.